Carrier owner gets to just fill their carrier over time knowing they've got a pretty safe 500k/t on a large inventory. Sure, they could get more if they sold directly instead of letting other players grab a trading profit, but that's the nature of commodity markets - would you rather have a certainty of 500k, or a chance of 1 million?
As a HODLER of Bitcoin since 2013, who added to his position in 2018 @ $4k BTC, I could sell that now for a certainty of $5k BTC profit, or HODL it until December 2021 for a chance of $245k profit.
With my fleet carrier I would always deal direct with Stations. Especially if my core mining time is the source. The only other metric is time, if I can do a few jumps in my FC to a far edge bubble system that sells LTD for 100k , the jump then the highest price system selling for 1.6m, I could in theory set up shop outside than station and sell my LTD for say 700k , 600k a ton profit for me, 1m ton profit for buyers, just a short buy sell trade route in station. But then why not just use my own Cargoed out T9/Corvette/Cutter/Python (medium pad only stations) to sell them direct myself ferrying the in system from FC to selling Station?